selftests: drv-net: Pull data before parsing headers

It is possible for drivers to generate xdp packets with data residing
entirely in fragments. To keep parsing headers using direct packet
access, call bpf_xdp_pull_data() to pull headers into the linear data
area.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250922233356.3356453-9-ameryhung@gmail.com